In all fields using a gas medium or a liquid medium such as air separation processes, petroleum refining, natural gas production, semiconductor devices manufacturing, specialty gas laboratories, etc . . . , all gas being processed or used in one way or another must be analyzed for quality control or process control. To perform such an analysis, a gas sample is collected and brought to an analytical measuring system. Generally, the gas sample is conveyed through metal tubing, up to a sample panel. A plurality of samples may need to be successively collected, depending on the complexity of a particular system. The analyzed sample should of course be representative of the gas medium being controlled.
The industry has used and still uses various devices and processes to bring a sample to an analytical system. With these sampling systems, contamination of a sample often occurs by mixing it with previously selected samples, leaks in or out of the sampling system or leaky valves.
Another unavoidable source of contamination are the control elements, i.e. valve, mass flow controller, pressure regulator, etc . . . , used to select and control flow of various sample streams. In U.S. Pat. No. 5,922,286 and in “Ppt level analysis of UHP Hydrogen”, European Semiconductor, 1996, it has been shown that any on-line component such as valve, mass flow controller, etc . . . , will act as a quasi continuous source of contamination when attempting to measure very low levels of impurities, i.e. ppb (part per billion) and ppt (part per trillion) level. These components will outgas some different types of molecules based on material used to manufacture them. When measuring particle contamination for process gas in semi-conductor industries, often, the limit of detection is limited by various types of control elements. Sudden change in flow and pressure generates wide variation in readings. For particle measurement, constant pressure flow is required. As indicated in the above mentioned U.S. Pat. No. 5,922,286 and as well known by people involved in the art, transient pressure or flow rate change system equilibrium, leading thus to an adsorption or desorption phenomenon. This lead to long purging time to recover system equilibrium.
Presently used in the art, there is a system provided with various sample lines made of various tubing material, each bringing a corresponding sample to an apparatus sample inlet. A plurality of sampling locations may be provided, as required by the process to be monitored. A bypass rotometer is provided in each line for purging a given sampling line when not selected. The rotometer allows fixing of a bypass flow and preferably sets a high flow in the sample line to speed up the purge time. The excess flow is vented out of the system. A female quick connector is provided at the extremity of each sampling line and is adapted to receive a male quick connector allowing the sample to flow through a flexible line up to the analytical system. To change the selected sample line, the male quick connector needs to be removed from the female quick connector and inserted in another one. This system makes sure that there is no sample cross contamination from various sample points, since the sampling lines are physically isolated. However, this system has serious drawbacks. First, each time the male quick connector is disconnected from a female quick connector, the gas flow to the analytical system is momentarily interrupted. Some analytical systems are affected by the sample flow variation. Also the female and male quick connectors have some internal dead volume that will be filled with atmospheric air when disconnected from each other. This air is directed to the analytical system and serious pollution may occur when measuring H2O, O2 or N2 as impurities in a particular background. Another drawback is that the quick connectors tend to wear out with use, resulting in leaks leading to wrong analytical results. Another problem with this system is related to the use of flexible tubing. Often this tube is made of various plastic or polymers that exhibit too much permeation to O2 and H2O, thereby polluting the sample. When flexible metal tubing is used, it must be replaced often since metal fatigue due to manipulation causes them to break. Another drawback is that such quick connectors have a o-ring that is used to seal them. The material used to make these o-rings will adsorb or desorb some of the impurities to be measured, so it changes the sample composition, making them a poor choice for low-level measurement.
Also used in the industry, there is another sample stream selection system quite similar to the one just described above. This second system uses, instead of quick connectors, a rotary selection valve well known in the industry and available from various manufacturers. This system alleviates some of the drawbacks of the previous one, but introduces cross port flow contamination that increases with time. If a sample line has a higher pressure, it will leak through a valve body and pollute the stream being measured. This valve requires frequent replacement. Furthermore, leaks can occur from the valve stem.
Also known in the art is another system wherein each sampling line includes an ON/OFF valve provided downstream a bypass rotometer. However, this system introduces dead volume in the line section downstream the valve. When switching from one sample to a new one, the line section of the previously selected sample is full of the previous sample. This gas is trapped there and will slowly diffuse in the line, slowing down the response time of the system and causing drifting readings of the analytical system. Another source of unswept dead volume is the valve itself. The space surrounding the valves plunger is always filled with sample gas and slowly diffuses in the main stream, causing measurement drift and noise. A Diaphragm based valve may be used to reduce the problem, but it increases the cost of the system since most of the time the use of such a valve will involve orbital welding for assembly. Furthermore, over time, ON/OFF valves will develop leaks. So an unselected stream may leak to a selected one, resulting in analytical error measurement and apparent drift or noise when the sample line pressure varies again. As soon as a valve develops a leak it must be replaced, interrupting the system in service. There are some variations of the previously described systems but all have similar drawbacks.
Also known in the art is the above mentioned U.S. Pat. No. 5,922,286 (Girard et al). Girard discloses a system that selects individual sample streams with the help of a 4-way, pneumatic actuated, VCR ¼″ connected diaphragm valve. Even if this system succeeds in eliminating unswept volume present on the discharge side of the valve and provides some means to have a sample inlet bypass flow or purge, it fails to eliminate the problem associated with leaking valves, i.e. crossport flow contamination. The selected sample must flow through all unselected valve bodies just around the seat, which is quite large. Therefore, the risk of crossport contamination increases with the number of sampling lines in the system. Diaphragms having a relatively short useful life, there will eventually be leaking across the seat and contamination of the selected sample will occur. Finally the diaphragm valves used in this system are costly and the total space required for this system is quite large.
Also known in the art is U.S. Pat. No. 6,637,277 by the same inventor of the present invention. In this patent, Gamache discloses a system that eliminates problems related to dead volume and leaking in valves by adding back purge flow on the discharge side of the valves, as illustrated in FIG. 1. Even if this proposed system corrects the inherent problems of all other previously described systems, the control valve (on-off valve) is still in contact with the process fluid to be analyzed, then contaminating it.
Other related prior art systems include U.S. Pat. Nos. 5,054,309; 5,055,260; 5,065,794; 5,239,856; 5,259,233; 5,447,053; 5,587,519 and 5,661,225.
In all prior art referred here, the sample to be introduced in the high sensitivity analytical system passes through the control element in some way. So, in all these cases the sample may be affected in some manner that leads to modify its level of impurities, leading to erroneous measurement.